Thyroid hemiagenesis (TH) is a rare, congenital condition defined by the absence of one of the two thyroid lobes. The thyroid gland is normally butterfly-shaped, comprising two lateral lobes connected by the isthmus. In TH, only the right or left lobe forms during fetal development, resulting in a significantly reduced volume of thyroid tissue. This condition is often discovered incidentally during imaging performed for unrelated reasons, as many affected individuals remain symptom-free. Understanding this anomaly requires examining its origins, diagnosis, functional status, and the long-term monitoring of the single remaining lobe.
Defining Thyroid Hemiagenesis and its Origins
The thyroid gland is the first endocrine gland to develop in the human embryo, beginning around the fourth week of gestation. Thyroid hemiagenesis results from a failure in the development of the thyroid primordium, where one lateral lobe fails to materialize during this early stage of organogenesis. This developmental error classifies the condition as a congenital malformation of the endocrine system.
The anomaly is rare, with an estimated prevalence ranging from 0.05% to 0.2% in the general population. The left lobe is far more frequently absent than the right, occurring in about 80% of reported cases. The isthmus, which connects the two lobes, is also absent in roughly half of the cases of hemiagenesis.
While the exact cause is often unknown, most cases are sporadic, though familial clustering suggests a possible genetic component. The condition may be associated with transcription factors, such as FOXE1 and PAX8, which are involved in the movement and formation of thyroid cells during embryogenesis. This developmental failure leads to an anatomically distinct, single-lobed gland.
Diagnostic Methods and Associated Findings
The diagnosis of thyroid hemiagenesis is an incidental finding made during imaging procedures. The initial and most common technique used to identify the condition is thyroid ultrasound. This non-invasive method allows for clear visualization of the neck anatomy, confirming the absence of one lobe and the isthmus, while assessing the size and structure of the remaining lobe.
To confirm the diagnosis, thyroid scintigraphy, or a nuclear scan, is frequently performed. This test involves administering a small amount of a radioactive tracer, such as technetium-99m, which is taken up by functioning thyroid tissue. The resulting image shows the tracer accumulating only in the single, present lobe, often creating a characteristic “hockey-stick” shape if the isthmus is absent.
A frequent associated finding is the compensatory hypertrophy of the existing lobe. The single remaining lobe enlarges to increase the total mass of functional thyroid tissue. This adaptation allows the gland to maximize hormone production, often maintaining a normal hormonal balance.
Clinical Implications and Functional Status
For most individuals, the single remaining thyroid lobe is fully capable of compensating for the missing tissue. The majority of patients with thyroid hemiagenesis remain euthyroid, meaning their thyroid-stimulating hormone (TSH) and thyroxine (T4) levels are within the normal range. The compensatory enlargement of the single lobe is a successful adaptation that prevents immediate hormonal imbalance.
Despite the euthyroid status, a significant clinical implication is the elevated lifetime risk of developing thyroid dysfunction. Patients with hemiagenesis are more likely to develop primary hypothyroidism later in life compared to the general population. This increased risk is thought to be due to the remaining lobe having a reduced functional reserve, making it more vulnerable to autoimmune attack or periods of increased hormonal demand, such as during pregnancy.
The remaining lobe has a higher incidence of associated pathologies. These conditions include the formation of benign nodules, multinodular goiter, and autoimmune thyroid diseases like Hashimoto’s thyroiditis or Graves’ disease. In rare instances, thyroid cancer, most often papillary thyroid carcinoma, has been found within the single lobe.
Long-Term Monitoring and Management
Once thyroid hemiagenesis is confirmed, the focus shifts to long-term care to detect any developing dysfunction or structural changes. The primary component of care involves periodic blood testing to monitor thyroid function. The recommendation is to check TSH and free T4 levels annually or biennially.
Routine imaging follow-up is important for the single lobe. Regular ultrasound surveillance allows physicians to track the size of the remaining lobe and monitor for the emergence of new nodules or changes in existing ones. This proactive screening is aimed at early detection and intervention, particularly if a suspicious nodule requires biopsy.
Treatment is not directed at the hemiagenesis itself, but rather at managing complications. If hypothyroidism develops, treatment involves hormone replacement therapy, usually with the synthetic thyroid hormone levothyroxine. Hormone replacement is also required if surgery removes the single remaining lobe due to cancer or severe pathology, resulting in lifelong dependence on external supplementation.